Team Kirkpatrick is using the episode to underline the charge that McCain is not the same maverick he once was.

“John McCain is once again putting his political party over Arizonans as he campaigns on East Coast radio stations and brags about refusing to do his constitutional duty,” said Kirkpatrick for Senate spokesperson D.B. Mitchell. “McCain isn’t a maverick or a straight talker and he showed today he will do anything to save his career, even if it means ignoring his constituents and the Constitution because after 33 years in Washington, he’s changed. Arizonans deserve better.”

Even if Ms. Clinton tapped someone considered more liberal than Mr. Garland, that nominee would deserve a fair hearing. Senators should accept presidential nominees unless they are either truly unqualified or true ideological extremists. The functioning of government depends on speedy and open-minded judicial confirmations. In the past, Mr. McCain, who ran for president in 2008 and recognizes the importance of a sound appointment process, was a voice of restraint on these matters. Now he recklessly encourages Republican voters to expect that GOP senators will refuse any Democratic Supreme Court nominee.

This is a dangerous road. If the Republicans keep the Senate majority next month, acting on such an expectation will establish the precedent that the judicial branch can be staffed only when the president and the Senate are of the same party. If the Democrats take control, GOP intransigence could lead them to quash the filibuster on Supreme Court nominees, which would further politicize the judiciary and poison the process.

Like many Republicans, Mr. McCain hit a professional low when he endorsed Republican presidential nominee Donald Trump this year, an endorsement he retracted only after seeing a video of Mr. Trump boasting about sexually assaulting women. His vow to further politicize the judicial nomination process is another sad marker.

If Clinton wins and Democrats pull enough Senate seats, Republicans will oppose her nominee, and then, eventually, Democrats will change the rules to abolish filibusters of Supreme Court nominees. (Republicans will decry this foul measure and justify any subsequent actions of their own as justified revenge.) If Clinton wins and Republicans hold on to 51 seats, they will simply refuse to let any nominee through. The fact that it is McCain, a personal friend of Clinton and as strong an institutionalist as can be found in the Senate, who is proposing to extend the blockade indefinitely shows just how deep the commitment runs through the party.

The old norms held that presidents were given some deference in filling Supreme Court vacancies. Senators might object to a particular nominee on the basis of ideological extremism or lack of qualifications, but the president’s general right to appoint a member of his judicial team was considered sacrosanct. Like all the other norms holding back the exercise of power, this one has now collapsed. The new rule is that a president needs 50 senators to fill a Supreme Court vacancy.